The invention relates to oil refinery operations, specifically to catalytic reforming of hydrocarbons, and more specifically to improved treatment of off-gases from the hydrocarbon reforming process.
The reformer is the unit that provides the octane level needed for the gasoline product of the refinery. The reformer is also the principal hydrogen producer within the refinery, and an important aspect of reformer operation is to generate as much hydrogen as possible, consistent with other requirements, of a quality suitable for use in the hydrogen-consuming units, particularly hydrocrackers and hydrotreaters.
The reformer feedstock is straight run naphtha or a naphtha cut from other refinery operations, such as coker naphtha or FCC (fluid catalytic cracking) naphtha. Although the bulk of components in straight run naphtha are paraffins, also present may be naphthenes, aromatics and olefins; non-virgin naphtha stocks tend to be higher in aromatics and olefins. Reforming involves the upgrading of these components to components of higher research octane number (RON) by various reactions. The process is generally carried out in three reaction zones, in each of which specific reactions are favored. For example, the first zone may perform, among other reactions, dehydrogenation of methylcyclohexane to toluene (RON 120), the second zone may perform dehydrocyclization of iso-heptane to toluene, and the third zone may perform isomerization of normal to iso-heptane (RON 90), as well as cracking of n-heptane to pentane (RON 90). Although the process is an overall producer of hydrogen, hydrogen is recycled back to the feed to maintain the hydrogen-to-hydrocarbon ratio in the reactors within a range to favor the desired reactions and to prolong the catalyst life. Typically hydrogen:hydrocarbon molar ratios up to about 10:1 are used.
In an ideal situation, no cracking that results in light hydrocarbons would take place, and the only by-product of the reactions would be hydrogen. In practice, this cannot be achieved. Conversion of iso-heptane to toluene and of methylcyclohexane to toluene are both hydrogen-producing reactions, but the cracking reactions result in formation of light hydrocarbon fragments, such as methane, ethane, propane and butanes. These light hydrocarbons contaminate the hydrogen product and may result in over-production of fuel gas. Under some reforming conditions, even more undesirable side reactions can give rise to formation of polycyclic aromatic compounds, some of which can be carried into the recycle gas. The net result is that, in an average refinery, 20,000 bbl of straight run naphtha feedstock may be converted into about 15,000 bbl of reformate and 5,000 bbl of gas, which includes substantial amounts of C1-C5 hydrocarbons.
The gaseous effluent from the reactor series is cooled and separated into liquid and vapor phases. The phase separation into liquid and vapor portions is often carried out in one or more steps by simply changing the pressure and/or temperature of the effluent. Therefore, in addition to hydrogen, the overhead vapor from the phase separation usually contains light hydrocarbons, specifically methane and ethane. In a closed recycle loop, these components build up, change the reactor equilibrium conditions, and can lead to reduced product yield. This build-up of undesirable contaminants is usually controlled by purging a part of the vapor stream from the loop. Such a purge operation is unselective however, and, since the purge stream may contain as much as 80 vol % or more hydrogen, multiple volumes of hydrogen can be lost from the loop for every volume of contaminant that is purged.
Since the reformer is a net hydrogen producer, the overhead vapor is typically split into at least two portions, one for recycle in the reactor loop, the other that forms a purge from the loop and that is frequently submitted to additional separation and treatment, such as by adsorption of the hydrocarbons. This creates a net hydrogen stream of a relatively high hydrogen concentration, such as 80%, 90% or above, for use elsewhere in the refinery, and a waste hydrocarbon stream to be sent for light ends recovery or to the fuel header. Use of pressure swing adsorption (ISA) for this upgrading step is known in the art and is taught, for example, in U.S. Pat. No. 5,332,492, in which tail gas is returned to the contactor section of the phase-separation steps, and U.S. Pat. No. 5,278,344, in which PSA is used to purify the net hydrogen stream after catalytic reforming and in front of a hydrodealkylation step.
It is also known to use membrane separation for removing hydrogen from hydrocarbons in refinery gas streams from various sources. U.S. Pat. Nos. 4,362,613, and 4,367,135, both to Monsanto, describe processes for treating the vapor from phase separators in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the reactor. U.S. Pat. No. 5 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane again permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L""Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from cracking effluent, the hydrogen separation being accomplished by a hydrogen-selective membrane. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen is and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal.
The use of certain polymeric membranes to treat off-gas streams in refineries is also described in the following papers: xe2x80x9cPrism(trademark) Separators Optimize Hydrocracker Hydrogenxe2x80x9d, by W. A. Bollinger et al., presented at the AICHE 1983 Summer National Meeting, August 1983; and xe2x80x9cOptimizing Hydrocracker Hydrogenxe2x80x9d by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. The use of membranes in refinery separations is also mentioned in xe2x80x9cHydrogen Technologies to Meet Refiners"" Future Needsxe2x80x9d, by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons. Papers that specifically concern treatment of reformer off-gases are xe2x80x9cHydrogen Purification with Cellulose Acetate Membranesxe2x80x9d, by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; and xe2x80x9cPlant Uses Membrane Separationxe2x80x9d, by H. Yamashiro et al., in Hydrocarbon Processing, February 1985. In these papers, a system and process using membranes to treat the overhead gas stream from the absorber/recontactor section of the plant are described. All of these papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons.
A chapter in xe2x80x9cPolymeric Gas Separation Membranesxe2x80x9d, D. R. Paul et al. (Eds.) entitled xe2x80x9cCommercial and Practical Aspects of Gas Separation Membranesxe2x80x9d, by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. Nos. 4,654,063 and 4,836,833, to Air Products and Chemicals, and U.S. Pat. No. 4,892,564, to Cooley. U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
It has also been recognized that condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843 and 5,374,300.
Numerous patents describe combinations of membrane separation with PSA. Representative examples include U.S. Pat. Nos. 4,229,188; 4,238,204; 4,398,926; 4,690,695; 4,701,187; and 4,783,203. U.S. Pat. No. 5,332,424, to Air Products and Chemicals, describes fractionation of a gas stream containing hydrocarbons and hydrogen using an xe2x80x9cadsorbent membranexe2x80x9d. The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Optionally, the membrane separation step is followed by PSA treatment. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,447,559; and 5,507,856, which all show combinations of carbon adsorbent membranes followed by PSA. U.S. Pat. No. 5,634,354 discloses removal of hydrogen from hydrogen/olefin streams. In this case, the membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen.
U.S. Pat. No. 5,435,836, concerns treatment of mixtures of hydrogen, carbon dioxide, carbon monoxide and methane from steam reformers. The gas mixture from the steam reformer is treated by PSA to recover a high purity hydrogen stream. The waste gas from the PSA unit is then treated by membrane separation using a carbon adsorbent membrane. The hydrogen-rich residue is returned to the PSA unit and the permeate gas from the membrane unit can optionally be used as fuel for the steam reformer. U.S. Pat. No. 5,753,010 discloses a process similar to that of U.S. Pat. No. 5,435,836, but in which the tail gas from the PSA unit is split into two fractions of unlike composition, which are treated separately in two discrete membrane steps.
U.S. Pat. No. 6,190,536 describes treatment of off-gases form fluid catalytic cracking absorbers using hydrocarbon-selective membranes.
U.S. Pat. No. 6,171,472 describes treatment of overhead gases in hydrocarbon conversion reactors of any type by passing gases in the reactor recycle loop across hydrocarbon-selective membranes. U.S. Pat. No. 6,190,540 describes such a process applied specifically to hydrotreaters and hydrocrackers.
U.S. Pat. No. 6,011,192 describes a process in which a rubbery polymeric membrane is used to condition a gas stream to remove heavy hydrocarbons before PSA treatment.
U.S. Pat. No. 6,165,350 describes the use of hydrogen-rejecting membranes to directly treat overhead gases from the phase separators of catalytic reformers.
The invention is an improved process and process train for catalytic reforming of hydrocarbons. In its most simple form, the invention includes four unit operations or steps: the reforming itself, usually carried out in a series of reactors; one or more steps to separate the reformate liquid product from overhead lighter components, predominantly C1-C6 hydrocarbons and hydrogen; one or more treatment steps to recover hydrogen from the overhead gases, and one or more treatment steps for the waste gas from the hydrogen recovery step. The invention also includes an improved catalytic reformer apparatus, comprising the reactor itself, the phase separation equipment, and equipment to treat the net gas produced by the reactors, this equipment including at least two different treatment units, the second of which is a membrane separation unit containing a polymeric membrane that is selective for C1-C6 hydrocarbons over hydrogen, and that treats the non-product gas from the first unit.
The catalytic reforming itself may be performed in any manner and using any types of reactors, catalysts and operating schemes known in the art. The separation step into liquid and overhead gas phases is typically, although not necessarily, carried out by first cooling the reactor effluent, producing a raw reformate liquid fraction, and a first overhead gas fraction, part of which is generally returned to the reforming reactors. This step may be followed by recontact of portions of the raw liquid and overhead gas phases at a lower temperature to promote additional absorption of heavier components into the liquid phase and partitioning of hydrogen and light gases into the gas phase. The treatment step for the remaining overhead gas, which is the source of net hydrogen produced by the reformer, involves separating the hydrogen from the hydrocarbons, and usually includes an adsorption step, typically by pressure-swing adsorption. The treatment steps for the waste gases produced when the adsorption beds are regenerated include compression/cooling, which may optionally result in the formation of an additional liquid fraction, and membrane separation using a polymeric membrane that is selective in favor of hydrocarbons over hydrogen.
In a basic embodiment, these steps take the following form:
(a) catalytically reforming a hydrocarbon feedstock in a reactor system;
(b) withdrawing an effluent stream comprising hydrogen and hydrocarbons from the reactor system;
(c) separating a reformate liquid phase and a vapor phase, comprising hydrogen and C1-C6 hydrocarbons, from the effluent stream;
(d) recirculating a portion of the vapor phase to the reactor system;
(e) passing at least a portion of the unrecirculated vapor phase through at least one treatment step, including an adsorption step carried out in an adsorption system, to separate hydrogen from the C1-C6 hydrocarbons;
(f) withdrawing a purified hydrogen product stream from the adsorption step;
(g) withdrawing a waste hydrocarbon stream from the adsorption system;
(h) compressing and cooling the waste hydrocarbon stream;
(i) passing at least a portion of the compressed, cooled waste hydrocarbon stream as a feed stream across the feed side of a polymeric membrane having a feed side and permeate side, and being selective for C1-C6 hydrocarbons over hydrogen;
(j) withdrawing from the feed side a residue stream enriched in hydrogen compared with the waste hydrocarbon stream;
(k) withdrawing from the permeate side a permeate stream enriched in C1-C6 hydrocarbons compared with the waste hydrocarbon stream.
Preferred embodiments include returning the hydrogen-rich stream from the membrane separation to the adsorption step, thereby increasing the amount of net hydrogen produced by the reformer. Preferred embodiments also include operating the compression/cooling step at pressure temperature combinations sufficient to liquefy the heavier of the hydrocarbon components present in the waste gas stream from the adsorption unit, thereby providing an additional LPG product stream.
A particularly preferred embodiment, therefore, takes the following form:
(a) catalytically reforming a hydrocarbon feedstock in a reactor system;
(b) withdrawing an effluent stream comprising hydrogen and hydrocarbons from the reactor system;
(c) separating a raw reformate liquid phase and a vapor phase, comprising hydrogen and C1-C6 hydrocarbons, from the effluent stream;
(d) recirculating a portion of the vapor phase to the reactor system;
(e) passing at least a portion of the unrecirculated vapor phase and at least a portion of the raw reformate liquid into a contactor;
(f) withdrawing from the contactor a gas stream depleted in C3+hydrocarbon content compared with the unrecirculated vapor phase;
(g) passing at least a portion of the gas stream through an adsorption step carried out in an adsorption system, to separate hydrogen from C1-C6 hydrocarbons;
(h) withdrawing a purified hydrogen product stream from the adsorption step;
(i) withdrawing a waste hydrocarbon stream from the adsorption system;
(j) compressing and cooling the waste hydrocarbon stream, thereby forming a condensate and an uncondensed portion;
(k) passing at least a portion of the uncondensed portion as a feed stream across the feed side of a polymeric membrane having a feed side and permeate side, and being selective for C1-C6 hydrocarbons over hydrogen;
(l) withdrawing from the feed side a residue stream enriched in hydrogen compared with the waste hydrocarbon stream;
(m) withdrawing from the permeate side a permeate stream enriched in the C1-C6 hydrocarbons compared with the waste hydrocarbon stream;
(n) recirculating at least a portion of the residue stream to the adsorption step.
Optionally, the hydrogen-rich stream from the membrane separation operation may be sent to a membrane purge step. This provides additional separation between the remaining light hydrocarbons and hydrogen, and enables C1 and C2 hydrocarbons to be purged from the reformer train with lower hydrogen loss than otherwise. In this aspect, the invention includes the following steps:
(a) catalytically reforming a hydrocarbon feedstock in a reactor system;
(b) withdrawing an effluent stream comprising hydrogen and hydrocarbons from the reactor system;
(c) separating a reformate liquid phase and a vapor phase, comprising hydrogen and the C1-C6 hydrocarbons, from the effluent stream;
(d) recirculating a portion of the vapor phase to the reactor system;
(e) passing at least a portion of the unrecirculated vapor phase through at least one treatment step, including an adsorption step carried out in an adsorption system, to separate hydrogen from the C1-C6 hydrocarbons;
(f) withdrawing a purified hydrogen product stream from the adsorption step;
(g) withdrawing a waste hydrocarbon stream from the adsorption system;
(h) compressing and cooling the waste hydrocarbon stream;
(i) passing at least a portion of the compressed, cooled waste hydrocarbon stream as a feed stream across the feed side of a polymeric membrane having a feed side and permeate side, and being selective for C1-C6 hydrocarbons over hydrogen;
(j) withdrawing from the permeate side a permeate stream enriched in C1-C6 hydrocarbons compared with the waste hydrocarbon stream;
(k) withdrawing from the feed side a residue stream enriched in hydrogen compared with the waste hydrocarbon stream;
(l) subjecting at least a portion of the residue stream to a membrane purge step, thereby forming a hydrogen-enriched stream and a light-hydrocarbon-enriched stream;
(m) purging the light-hydrocarbon-enriched stream from the process; and optionally,
(n) recirculating at least a portion of the hydrogen-enriched stream to the adsorption step.
In yet another aspect, the invention is an improved catalytic reformer train, including the reforming reactor, liquid/vapor phase separators, an adsorption system, and a membrane separation system including a compression/cooling unit and a membrane unit.
The invention has a number of advantages, including but not limited to:
increased hydrogen production compared with prior art techniques
production of a discrete LPG stream
ability to debottleneck plants where fuel gas production is at maximum
preparation of feed streams for steam reforming.
Polymeric materials are used for the membranes. This renders the membranes easy and inexpensive to prepare and to house in modules, by conventional industrial techniques, unlike other types of hydrogen-rejecting membranes, such as finely microporous inorganic membranes, including adsorbent carbon membranes, pyrolysed carbon membranes and ceramic membranes, which are difficult and costly to fabricate in industrially useful quantities.
The use of hydrocarbon-selective, hydrogen-rejecting polymeric membranes means that the hydrogen-enriched stream is retained on the feed side of the membrane. In other words, the hydrogen-enriched stream withdrawn from the membrane separation unit remains at pressure, which is desirable for recycle to the adsorption unit, as well as for facilitating delivery to most other destinations.